This invention relates to oxygen therapy apparatus for supply of breathable gas, for example, oxygen to patients.
Oxygen therapy is widely used in medical applications and is very widely applied in hospitals, with oxygen therapy capability to most hospital beds. However, many known oxygen therapy flow devices waste up to ⅔ of the oxygen delivered by the device due to the fact that the device delivers flow during the period when the patient is exhaling. There is further wastage, as only the oxygen delivered at the start of a breath goes deep into the lungs where it is absorbed. Furthermore, the patient""s need and the available settings are often poorly matched; for example a person needing 2.5 L/m might have to have 4 L/m, because of the small number of settings available. The invention thus relates in particular to an oxygen economiser device for oxygen therapy apparatus which device seeks to reduce this wastage of oxygen.
Many patients are dependent on oxygen for mobility, and so have to carry cylinders lasting typically a couple of hours. An oxygen economiser device can be used to make the same cylinder last longer, or make a much smaller and lighter cylinder meet the existing time.
There are a number of oxygen economiser devices on the market, working by one of two ways. A first is an electrically operated device which builds up a reservoir of gas during exhalation and, as the patient starts to inhale, opens a valve for a moment, giving a pulse of oxygen into the first part of the inhaled breath. Variation in delivery is given by operating the unit every other breath, every third breath or every fourth breath. This saves the oxygen, but has a limited number of settings, and needs batteries and associated circuitry. Additionally such units normally have additional controls which are undesirable for the people using oxygen therapy.
A second device is one which incorporates what is effectively a demand valve with a diaphragm to detect the decrease in pressure on inhalation which (by pilot operation, say) opens a valve for the main flow, and closes it when exhalation ceases. This type of device has to have a twin tube supplying the patient, since the resistance of the tube during flow (say 500 mm H2O) is many times the magnitude of the signal, so is too great to allow the slight negative pressure signal (say 3 mm H2O) to the diaphragm, so would close the diaphragm.
According to the invention there is provided a pneumatically operated economiser device for supply of breathable gas to a patient, said device having an inlet port for receiving a supply of pressurised gas and an outlet port for delivering a supply of pressurised gas to the patient, valve means between the inlet port and the outlet port, said valve means being switchable between a first position in which flow of gas from the inlet port to the outlet port is prevented, and a second position in which gas may flow from said inlet port to said outlet port, means for monitoring for inhalation by the patient, actuator means normally maintaining the valve means in said first position but switching said valve means to said second position when the pressure at the outlet port, as detected by said monitoring means, falls below a preset level indicative of inhalation, and delay means for maintaining the valve means in said second position for a preset period.
Generally speaking, the breathable gas will be oxygen and, for convenience, this will be assumed throughout the following description. However gases other than oxygen, and mixtures of oxygen with other gases or vapours are possible.
Preferably said preset period is less than the expected period of inhalation and advantageously it is considerably shorter than the inhalation period. A typical preset period is about 0.5 seconds.
The monitoring means is preferably operable to continuously monitor the outlet pressure during exhalation, and to sample it at regular spaced intervals sufficiently small not to cause delay in the switching of the valve means to the open position during inhalation.
At the end of the preset delay period, the valve means reverts to the closed condition but if, at this point, the monitoring means is still indicating that inhalation is taking place, the actuator means will immediately re-open the valve to allow the supply of oxygen to recommence. This opening and closing of the valve means will continue until such time as, at the end of a preset delay period, the monitoring means indicates that exhalation has commenced. During exhalation, the valve means remains in said first positionxe2x80x94i.e. closed.
Thus, during inhalation, the patient receives a pulsed flow of oxygen having a period equal to the aforementioned preset period. During exhalation no oxygen is supplied, thus resulting in a significant saving of oxygen over conventional oxygen therapy devices which do not use oxygen economiser techniques.
This invention allows the use of single tube cannulas and single tube is face masks, as used in conventional (non oxygen economiser) oxygen therapy devices. Known oxygen economiser devices make use of two tubes leading to the patient, one to supply the oxygen, and one to monitor the status of the breathing cycle. The device of the invention is thus able to utilise existing (single tube) cannulas which are more comfortable for the patient, and do not result in supply being tied to a particular manufacturer.
In the present invention the outlet port is two way, and therefore both transmits the outgoing oxygen to the patient, and receives a pressure signal resulting from the exhaled breath from the patient. The monitor is operable to monitor the pressure at the outlet port and thus inevitably monitors both the pressure of oxygen during inhalation and the pressure of exhalation. Four conditions at the outlet port can be identified:
1) The valve means is closed and exhalation is taking place. In this case the monitored pressure is likely to be relatively high, thus causing the actuator means to maintain the valve means in the closed condition.
2) The valve means is closed and inhalation is taking place. In this case the monitored pressure is relatively low, and probably slightly negative with respect to atmospheric, and this causes the actuator means to open the valve means for the preset period.
3) The valve means is open and exhalation is taking place. This condition is possible only when the delay means is maintaining the valve means open for the preset period and, during the period, the patient has switched from inhalation to exhalation. Once the valve means closes at the end of the preset period, the condition will revert to (1) above and will remain so until inhalation recommences.
4) The valve means is open and inhalation is taking place. In this case the monitored pressure is relatively high due to the pressure of oxygen being supplied to the patient but, despite this, the valve remains in the open condition for the remainder of the preset period.
It will be seen from the above that it is important with the xe2x80x9csingle tubexe2x80x9d arrangement that the supply of oxygen is shut off at regular intervals during inhalation in order to allow the monitoring means to check for continued inhalation. During supply of oxygen to the patient the small negative pressure of inhalation is swamped by the pressure of the oxygen itself and it is not until the supply is halted that the monitor means is able to properly detect whether the patient is inhaling or exhaling.
Preferably said valve means comprises a movable member movable between a first position in which the valve means is closed and a second position in which the valve means is open and wherein said actuator means is operable upon sensing inhalation, to move said movable member from said first position to said second position and wherein said delay means is operable to cause said movable member to move back from said second position to said first position over a period equal to said pre-set period.
The movable member can take a number of forms, for example, a diaphragm or a piston. In a preferred embodiment of the invention, the movable member takes the form of a piston which is movable within a cylinder and is subject to balancing forces as between a biassing means, for example a spring on the one hand and pressure from said actuator means on the other. Preferably the pressure from the actuator means is gas pressure applied to the opposite side of said piston to the biassing means. Thus said actuator means comprises means for altering the gas pressure applied to said piston which results in movement of said piston, against the force of said biassing means, from said first position to said second position, or vice versa. For example, in one embodiment, the means for altering the gas pressure is operable to increase the gas pressure on said opposite side of the piston, thus moving the piston against the force of said biassing means, this movement being, in this case from the first (closed) position to the second (open) position. In another embodiment, the means for altering the gas pressure is operable to reduce the gas pressure on said opposite side of the piston, thus allowing the piston to move by the force of said biassing means, this movement being in this case from the first (closed) position to the second (open) position.
Preferably the means to alter the gas pressure comprises a further valve means operable to supply or withdraw gas pressure to or from said opposite side of said piston. Said valve means may be of any suitable type, for example a diaphragm valve or a piston-operated valve.
Thus, in one embodiment, means are provided for pressurising said opposite side of said piston, against the force of said biassing means, with sufficient pressure into said first position to normally maintain the first-mentioned valve means in the closed position, thereby cutting off the oxygen supply to the patient. A reduction in pressure at the outlet, indicative of inhalation, causes said further valve means to open which vents the opposite side of the piston, thus reducing the pressure and allowing the first-mentioned valve means to switch to the open position. This in turn results in an increased pressure at the outlet port, which increased pressure causes said further valve means to close again, thus cutting off the vent. Meanwhile, said pressurising means continues to supply gas to the opposite side of the piston so that, after a period governed by the speed at which the pressurising means is able to introduce gas to said opposite side of the piston, the first-mentioned valve means closes again, thus cutting off the supply to the patient, and the cycle repeats. This is explained in more detail hereinafter. Preferably the pressurising means incorporates a flow restrictor so as to increase this period. In practice, a period of about 0.5 seconds is typical.
In an alternative embodiment, the actuator means comprises two valves which act in tandem: A first valve acts to sense the pressure at the outlet port. A second valve is operable to switch the application of gas pressure to said opposite side of said piston. In this case, the first valve, which may for example be a diaphragm valve, actuates the second valve, which may for example be a piston-operated valve, to supply gas pressure to the opposite side of the piston of said first mentioned valve means, or not, as the case may be. The exact interrelationship of the first and second valves, and the first-mentioned valve means, is explained in more detail hereinafter.
A reservoir can be incorporated in the supply of gas to the first mentioned valve means in order to provide a pulse of increased pressure at the beginning of the inhalation period. The reservoir is preferably supplied via a flow restrictor, so that the reservoir and flow restrictor operate in tandem to enable the characteristics of the increased pressure at the beginning of inhalation to be tailored to requirements.